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直击本质：WPF 框架是如何实现模态窗口的

news 来源：原创 2024/5/2 19:12:03

想知道你在 WPF 编写 Window.ShowDialog() 之后，WPF 框架是如何帮你实现模态窗口的吗？

本文就带你来了解这一些。

本文内容

- `Window.ShowDialog`
- `ShowHelper`

`Window.ShowDialog`

WPF 显示模态窗口的方法就是 Window.ShowDialog，因此我们直接进入这个方法查看。由于 .NET Core 版本的 WPF 已经开源，我们会使用 .NET Core 版本的 WPF 源代码。

Window.ShowDialog 的源代码可以在这里查看：

Window.cs

这个方法非常长，所以我只把其中与模态窗口最关键的代码和相关注释留下，其他都删除（这当然是不可编译的）：

public Nullable<bool> ShowDialog()
{
    // NOTE:
    // _threadWindowHandles is created here.  This reference is nulled out in EnableThreadWindows
    // when it is called with a true parameter.  Please do not null it out anywhere else.
    // EnableThreadWindow(true) is called when dialog is going away.  Once dialog is closed and
    // thread windows have been enabled, then there no need to keep the array list around.
    // Please see BUG 929740 before making any changes to how _threadWindowHandles works.
    _threadWindowHandles = new ArrayList();
    //Get visible and enabled windows in the thread
    // If the callback function returns true for all windows in the thread, the return value is true.
    // If the callback function returns false on any enumerated window, or if there are no windows
    // found in the thread, the return value is false.
    // No need for use to actually check the return value.
    UnsafeNativeMethods.EnumThreadWindows(SafeNativeMethods.GetCurrentThreadId(),
                                            new NativeMethods.EnumThreadWindowsCallback(ThreadWindowsCallback),
                                            NativeMethods.NullHandleRef);
    //disable those windows
    EnableThreadWindows(false);

    try
    {
        _showingAsDialog = true;
        Show();
    }
    catch
    {
        // NOTE:
        // See BUG 929740.
        // _threadWindowHandles is created before calling ShowDialog and is deleted in
        // EnableThreadWindows (when it's called with true).
        //
        // Window dlg = new Window();
        // Button b = new button();
        // b.OnClick += new ClickHandler(OnClick);
        // dlg.ShowDialog();
        //
        //
        // void OnClick(...)
        // {
        //      dlg.Close();
        //      throw new Exception();
        // }
        //
        //
        // If above code is written, then we get inside this exception handler only after the dialog
        // is closed.  In that case all the windows that we disabled before showing the dialog have already
        // been enabled and _threadWindowHandles set to null in EnableThreadWindows.  Thus, we don't
        // need to do it again.
        //
        // In any other exception cases, we get in this handler before Dialog is closed and thus we do
        // need to enable all the disable windows.
        if (_threadWindowHandles != null)
        {
            // Some exception case. Re-enable the windows that were disabled
            EnableThreadWindows(true);
        }
    }
}

觉得代码还是太长？不要紧，我再简化一下：

EnumThreadWindows 获取当前线程的所有窗口
把当前线程中的所有窗口都禁用掉（用的是 Win32 API 的禁用哦，这不会导致窗口内控件的样式变为禁用状态）
将窗口显示出来（如果出现异常，则还原之前禁用的窗口）

可以注意到禁用掉的窗口是“当前线程”的哦。

`ShowHelper`

接下来的重点方法是 Window.ShowDialog 中的那句 Show()。在 Show() 之前设置了 _showingAsDialog 为 true，于是这里会调用 ShowHelper 方法并传入 true。

下面的代码也是精简后的 ShowHelper 方法：

private object ShowHelper(object booleanBox)
{
    try
    {
        // tell users we're going modal
        ComponentDispatcher.PushModal();

        _dispatcherFrame = new DispatcherFrame();
        Dispatcher.PushFrame(_dispatcherFrame);
    }
    finally
    {
        // tell users we're going non-modal
        ComponentDispatcher.PopModal();
    }
}